This article is adapted from “A Very Stable Genius: Donald J. Trump’s Testing of America,” which will be published on Jan. 21 by Penguin Press.

There is no more sacred room for military officers than 2E924 of the Pentagon, a windowless and secure vault where the Joint Chiefs of Staff meet regularly to wrestle with classified matters. Its more common name is “the Tank.” The Tank resembles a small corporate boardroom, with a gleaming golden oak table, leather swivel armchairs and other mid-century stylings. Inside its walls, flag officers observe a reverence and decorum for the wrenching decisions that have been made there.

Hanging prominently on one of the walls is The Peacemakers, a painting that depicts an 1865 Civil War strategy session with President Abraham Lincoln and his three service chiefs — Lieutenant General Ulysses S. Grant, Major General William Tecumseh Sherman, and Rear Admiral David Dixon Porter. One hundred fifty-​­two years after Lincoln hatched plans to preserve the Union, President Trump’s advisers staged an intervention inside the Tank to try to preserve the world order.

By that point, six months into his administration, Secretary of Defense Jim Mattis, Director of the National Economic Council Gary Cohn, and Secretary of State Rex Tillerson had grown alarmed by gaping holes in Trump’s knowledge of history, especially the key alliances forged following World War II. Trump had dismissed allies as worthless, cozied up to authoritarian regimes in Russia and elsewhere, and advocated withdrawing troops from strategic outposts and active theaters alike.

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Trump organized his unorthodox worldview under the simplistic banner of “America First,” but Mattis, Tillerson, and Cohn feared his proposals were rash, barely considered, and a danger to America’s superpower standing. They also felt that many of Trump’s impulsive ideas stemmed from his lack of familiarity with U.S. history and, even, where countries were located. To have a useful discussion with him, the trio agreed, they had to create a basic knowledge, a shared language.

So on July 20, 2017, Mattis invited Trump to the Tank for what he, Tillerson, and Cohn had carefully organized as a tailored tutorial. What happened inside the Tank that day crystallized the commander in chief’s berating, derisive and dismissive manner, foreshadowing decisions such as the one earlier this month that brought the United States to the brink of war with Iran. The Tank meeting was a turning point in Trump’s presidency. Rather than getting him to appreciate America’s traditional role and alliances, Trump began to tune out and eventually push away the experts who believed their duty was to protect the country by restraining his more dangerous impulses.